Make all values of array Nan except for specified locations

I have a 2d matrix 2600x7200 and i have the locations in this matrix that i need to extract. There are 19602 of these locations.

I need a way to either make every location other than the 19602 become Nan or a way to extract these specific locations to another array of zeros.

Any help on this topic would be much appreciated

Answer 1

[Fangjun Jiang]

%%
a=magic(5);
RowPos=[1,3,5];
ColPos=[2,4,5];
ind=sub2ind(size(a),RowPos,ColPos);
b=nan(size(a));
b(ind)=a(ind);

[Jordan Coombs]

Thank you so much, was struggling with this for ages!